Variations From
Ideal Geometry

Many factors lead to
variations from the ideal bond angles of a molecular shape. Size of the atoms
involved, presence of lone pairs, multiple bonds, large groups attached to the
central atom, and the environment that the molecule is found in are all common
factors to take into consideration. There are molecules in the database that
are good examples of all of these things.

Suggested
activity #1:

If students did activity #1 in the "VSEPR"
topic, they will have already noticed that although there are specific angles
that create an ideal tetrahedron or trigonal planar molecule, molecules of
those shapes don't often possess angles of exactly 109.5° or 120°. They will probably already
be wondering why that is, which can lead directly into a discussion of the
additional repulsion created by a lone pair, multiple bond, or an atom that is
relatively large in comparison to the others in the molecule.

Giving students a few molecules to
look at that display non-ideal angles and asking them to come up with an
explanation would be a great exercise. It is similar to activity #2 in the
"VSEPR" topic, and is based on the same principle of atomic repulsion. This
exercise will also give students practice in drawing lewis structures because they
will need to decide if a multiple bond is present before they can correctly
explain why the molecule does conform to ideal geometry.

In the case above, the bond
angle is quite a bit smaller than the 120° that would be predicted
based on the hybridization (sp2, which would normally indicate a trigonal
planar molecule) because of the lone pair of electrons on the nitrogen. The
lone pair occupies more space around the central nucleus than a bond would
because the orbital is not stretched between two nuclei as it would be in a
bond. As a result, two N-O bonds are forced closer together more than they
would be in a trigonal planar molecule.